This invention relates to an electroluminescent device comprising at least one electroluminescent organic layer including a light emitting material layer interposed between an anode and a cathode, wherein the light emitting material layer emits light upon application of a voltage between the anode and the cathode.
The electroluminescent phenomenon of organic material was observed on anthracene single crystals (J. Chem. Phys., 38 (1963), 2042). Thereafter, a relatively intense luminescent phenomenon was observed using a solution electrode having high injection efficiency (Phys. Rev. Lett., 14 (1965), 229). Thereafter, active research works were made on organic luminescent materials between conjugated organic host materials and conjugated organic activators having a fused benzene ring (U.S. Pat. Nos. 3,172,862, 3,173,050, 3,710,167, J. Chem. Phys., 44 (1966), 2902, and J. Chem. Phys., 50 (1969), 4364). The organic luminescent materials listed herein, however, suffer from the drawbacks of increased film thickness and a high electric field needed to induce luminescence.
As one countermeasure, researches were made on thin-film devices using evaporation technique and succeeded in lowering drive voltage. Such devices, however, failed to provide luminance at a practically acceptable level (Polymer, 24 (1983), 748, and Jpn. J. Appl. Phys., 25 (1986), L773).
Recently, Eastman Kodak proposed a device in which a charge transporting layer and a light emitting layer are formed between electrodes by an evaporation technique, accomplishing a high luminance at a low drive voltage (Appl. Phys. Lett., 51 (1987), 913 and U.S. Pat. No. 4,356,429). Thereafter, research works were further activated, as by shifting to three layer type devices in which carrier transporting and light emitting functions are separated. From then onward, the study on organic electroluminescent devices entered the practical stage (Jpn. J. Appl. Phys., 27 (1988), L269, L713).
However, there remains a serious problem of product lifetime as demonstrated by a luminescent life which is 3,000 hours at the shortest and several ten thousands of hours at the longest when operated at several hundreds of candelas.
It was also found that the above-described devices are prone to delamination due to moisture adsorption and thermal degradation and substantially increase dark spots during long-term service. It is believed that such degradation is mainly caused by interfacial separation between the inorganic electrode and the organic layer and the potential barrier between the electrodes and the respective carrier transporting materials although these problems remain outstanding.
Therefore, an object of the invention is to provide an organic electroluminescent device which is restrained from thermal degradation and has improved heat resistance and durability.
Making extensive investigations to attain the above object, the inventors have found that in an electroluminescent device comprising at least one electroluminescent organic layer interposed between the anode and the cathode, especially an electroluminescent device in which an organic hole transporting layer and a light emitting material layer are sequentially deposited on an inorganic electrode (ITO electrode etc.) serving as the anode and the cathode is disposed thereon, improved adhesion and durability are achieved by providing an auxiliary carrier transporting layer between the anode and the organic layer (especially between the inorganic electrode and the organic hole transporting layer), and forming the auxiliary carrier transporting layer from a soluble, electrically conductive compound or polymer in the form of a salt that the polyimide precursor and/or polyimide defined below forms with an electron accepting dopant.
Specifically, the invention provides an electroluminescent device comprising an anode, a cathode, and at least one electroluminescent organic layer interposed therebetween, wherein a luminescent material in the organic layer emits light upon application of a voltage between the anode and the cathode, characterized in that a salt of an electron accepting dopant with a polyimide precursor and/or polyimide having oligo-aniline units on side chains is formed as an auxiliary carrier transporting layer between the anode and the organic layer, the polyimide precursor and/or polyimide being obtained from a diamine component containing at least 1 mol % of an oligo-aniline unit-bearing diaminobenzene derivative represented by the following general formula (1) and a tetracarboxylic dianhydride or derivative thereof. 
Herein R1 to R9 each are independently hydrogen, an alkyl group, an alkoxy group, a sulfonate group, or a substituted or unsubstituted cyclohexyl, biphenyl, bicyclohexyl or phenylcyclohexyl group, n is a positive number of at least 1, A is a single bond or a divalent organic group selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94CONHxe2x80x94 and xe2x80x94NHxe2x80x94, and R0 is a trivalent organic group containing an aromatic ring.
According to the invention, an aniline oligomer is attached to the polyimide backbone as a side chain, and the aniline oligomer is doped with a halogen, Lewis acid, protonic acid, transition metal compound or electrolyte anion to impart electric conductivity for acquiring an electrode function. This improves the adhesion between the inorganic electrode and a hole transporting layer which is the organic layer while maintaining a hole transporting capability, and precludes interfacial phenomena such as separation for thereby improving the durability of the device itself.